1. Field of the Invention
This invention relates to a small sized high voltage transformer comprising a primary coil, a secondary coil, and a magnetic core, and to an ignition transformer using the high voltage transformer.
2. Description of the Related Art
Transformers used in automobiles for ignition, and flyback transformers for driving cathode ray tubes are required to generate a high pulse voltage output of 10 kV to 35 kV. These transformers have firstly been assembled from a primary coil, a secondary coil and a magnetic core, and then a casting resin is injected into the coil part, and subsequently cured to complete the constitution of a transformer. In this type of transformers a high voltage is produced in the secondary coil by raising a special pulse voltage fed into the primary coil.
Described below is a detailed example of known manufacturing method for an ignition transformer of direct ignition type used for automobiles as illustrated in FIG. 1. A secondary coil 3 wound on a secondary bobbin 2 is arranged around an inner magnetic core 1-1. After a primary coil 5 wound on a primary bobbin 4 is arranged and an inner magnetic core 1-2 is attached to the both ends of the coil, the parts are housed in a case 6. A casting resin 7 is poured into the case 6 to fill the clearance in a transformer 8 and the void in coil 9, and subsequently heat cured. Putting an exterior magnetic core 1-3 around the case completes the constitution of the transformer.
An example of conventional method widely used for manufacturing a flyback transformer for driving cathode ray tubes as shown in FIG. 2 is also described below. A secondary coil 3 wound on an intermediate layer 10 coated on the secondary bobbin 2 is arranged around a primary coil 5 wound on a primary bobbin 4. The parts are housed in a case 6. A casting resin 7 is injected into the case 6 to fill the clearance in the transformer 8 and the narrow void in coil 9, and then heat cured. Fitting of the magnetic core 1 completes the constitution of the transformer.
These transformers are required to function properly for a long period of use life under a high temperature in a cramped space in the transformer. Therefore, long term durability under heat and moisture has been a very important requirement. In the manufacture of such high voltage transformers, choice of combination of the casting resin and the material used for the bobbin is very important. The reason is that the lack of adhesion between the two materials may cause separation of the two materials. Difference in heat expansion coefficients between the two materials may case thermal stress, resulting in cracking in the casted resin. Thereby the dielectric breakdown in the coil may occur due to the electric discharge. In addition, withstanding voltage properties of the bobbin material and the casting resin are also required.
To avoid dielectric breakdown, attempts have been made to select a combination of a casting resin and a bobbin material which will give good adhesion between the two materials. For this reason, epoxy resins having a heat distortion temperature ranging from 90.degree. C. to 120.degree. C. have been widely used as the casting resins in combination with a bobbin material such as a blend of polyphenylene oxide and polystyrene (ex. Noryl, Trademark of GE Company) having heat distortion temperature of approximately 120.degree. C. The reason why the above combination has been selected lies in the belief that surface of Noryl resin partially swells when contacted with a liquid epoxy resin thereby providing a good adhesion layer as the epoxy resin undergoes curing.
However, heat distortion temperatures of epoxy resins and bobbin materials conventionally used are not high enough. Therefore, these materials tend to soften when transformers are subjected to a temperature higher than 120.degree. C. This has been a cause of mechanical deformation and dielectric breakdown of the materials employed in transformers.
In the conventional distributor system, one transformer is connected to multiple number of engines. On the other hand, recently, in order to improve power controllability of automobiles, a direct ignition system has been adopted, wherein plural transformers are connected directly to the same number of engines.
As a flyback transformer for driving cathode ray tubes, weight reductions of display is becoming major requirement in the market as well as the requirement for cost reduction. In these types of transformers, reduction in weight, size and cost of transformers are important issues.
However, in the conventional transformer, since the combination of materials were limited, and could not satisfy the severe requirement for use and its size reduction. When an epoxy resin of higher heat distortion temperature is used to improve heat resistance in combination with a conventional bobbin material, matching of heat expansion coefficients of the two materials becomes a problem resulting in poor adhesion between the two materials.